Sodium polyacrylate is a superabsorbent polymer utilized in household items such as diapers, artific — Polymers / Physical Properties / Osmosis Chemistry Question
Problem Context
Sodium polyacrylate is a superabsorbent polymer utilized in household items such as diapers, artificial snow, and water absorbing spheres.
Using the provided materials, Rank the aqueous solutions provided in order of their ability to be absorbed by sodium polyacrylate.
Give a brief description of your experimental plan.
Model Answer
Excellent plan indicates a strategy to determine the absorption properties of the polymer
- use a standard amount of polymer (either spatula full or some mass amount) for each trial
- add small quantities of each solution and observe the addition of solution to the polymer
- determine in terms of drops or volume the amount of solution absorbed
Alternative excellent plan includes
- use a standard volume of solution for each trial
- add small quantities of polymer to the solution until the mixture solidifies
- quantify the amount of polymer added
Replicate measurements
Record your data and other observations.
Model Answer
Starting with approximately the same amount of polymer (spatula full) and counting the amount of drops added to the sample before the polymer no longer absorbs the solution
Solution | Number of Drops
Ammonium chloride | 6-7 drops
Magnesium chloride | 3-4 drops
Sodium chloride | 8 drops
water | 26-27 drops
Glucose | 30-32 drops
Alternative trial
Solution | Mass of polymer before adding solution | Mass of polymer plus solution (added until saturation) | Mass of solution absorbed per gram of polymer
Ammonium chloride | 0.068 g | 7.427 g | 110
Magnesium chloride | 0.068 g | 3.467 g | 50
Sodium chloride | 0.067 g | 7.275 g | 110
Water | 0.066 g | 27.84 g | 410
Glucose | 0.067 g | 21.09 g – ran out of 25 mL solution | >310
The order of absorption of the aqueous solutions by the polymer is:
Least absorbed Most absorbed
Model Answer
Least absorbed most absorbed
Mg2+ (aq) < NH4+ (aq) ≤ Na+ (aq) < glucose ≤ water
Explain the observed trend in the data using chemical principles.
Model Answer
Water is absorbed by the polymer through osmotic swelling of the gel. When water is added to the polymer network, the sodium ions distribute equally between the network and the solution. To maintain the osmotic pressure inside and outside the network, the water molecules enter and swell the polymer network to keep the sodium concentration balanced between the water and polymer. The cations are attracted to the negative carboxylates in the backbone and the structure collapses or absorbs less water. The degree of swelling is proportional to the ionic charge, radius and the degree of hydration. An alternative explanation about the lower swelling with the divalent cation is that the divalent cation increases the cross-linking density of the polymer and reduces the equilibrium swelling.